1. Field of the Invention
This invention relates to a liquid crystal composition for liquid crystal displays. More particularly it relates to a liquid crystal composition used for high multiplex display devices making use of supertwisted birefringence effect mode.
2. Description of the Related Art
With recent rapid development of information devices, particularly growth of portable terminal devices, display devices of small size, thin shape and low power consumption, having a display capacity and a display quality matching those of conventional CRT have been required. In response to the above-mentioned needs for small size, thin shape and lower power consumption, liquid crystal devices capable of multiplex drive by means of TN type liquid crystal cells having a twist angle of 90.degree., have so far been used in the form of display terminals up to about 1/100 duty for watches, electronic calculators, etc. However, an increase of the duty number to more than the above has been regarded as difficult in principle, since it reduces the display quality.
Whereas, there has been known a supertwisted birefringence effect mode making use of the birefringence effect of liquid crystal cells, having set the twist angle to 180.degree.-270.degree., at which the bistability in the electrooptical characteristic of chiral nematic liquid crystals occurs to the utmost. This mode is a supertwisted birefringence effect mode using an organic aligned membrane, and hereinafter abbreviated to SBE mode, including modes called STN mode and HBE mode. According to this SBE mode, the same drive method as in the matrix display of conventional TN mode of 90.degree. twist, that is, the matrix display driven according to the first scan-addressing mode replying to the effective value of impressed voltage is possible, and also a considerably better contrast and a broader viewing angle than those in the case of TN display of 90.degree. twist can be afforded (T. J. Scheffer, J. Nehring; Appl. Phys. Letter, 45, 1021 (1984)).
With reference to liquid crystal materials, in order to make the voltage-transmittance characteristic curve steep in the case of conventional 90.degree. twist TN mode, it is necessary to make the elastic constant ratio K.sub.33 /K.sub.11 of liquid crystal materials as low as possible. This is evident for example from the report of G. Baur {The Physics and Chemistry of Liquid Crystal Devices (edited by G. J. Sprokel), pp 61-78 (1980)}. However, in order to make the voltage-transmittance characteristic curve steep in the case of a SBE mode, it is necessary to make the ratio K.sub.33 /K.sub.11 as large as possible, contrary to the case of 90.degree. twist TN mode. Further, it is preferred that the ratio K.sub.33 /K.sub.22 also be larger. This is evident e.g. from the report of C. M. Waters (Design of Highly Multiplexed Liquid Crystal Dye Displays, Mol. Cryst. Liq. Cryst., 1985, vol. 123, pp 303-319). Here, K.sub.11, K.sub.22 and K.sub.33 represent a splay elastic constant, a twisted one and a bent one, respectively, in the continiuum theory (F. Frank: Dix. Faraday Soc., 25, p 19 (1956)).
Namely, as described above, the physical property value for making the voltage-transmittance characteristic curve steep, i.e. the elastic constant ratio K.sub.33 /K.sub.11 in the case of a 90.degree. twist TN mode is quite contrary to that in the case of SBE mode. Thus, it can be seen that liquid crystal compositions used so far in the 90.degree. twist TN mode are unsuitable for making the voltage-transmittance characteristic curve steep in the SBE mode.
The main object of the present invention is to provide a liquid crystal composition suitable for liquid crystal display elements of SBE mode for making the voltage-transmittance characteristic curve steep, taking into account the above-mentioned theoretical background and the practical needs for a liquid crystal display element.
Characteristics required for liquid crystal compositions used for display elements of the SBE mode are as follows:
(1) steep voltage-transmittance characteristic curve; PA0 (2) high nematic-isotropic liquid phase transition point; PA0 (3) capability of exhibiting a suitable optical anisotropy value (hereinafter abbreviated to .DELTA.n) in accordance with the cell thickness (hereinafter abbreviated to d); PA0 (4) low viscosity (hereinafter abbreviated to .eta.); etc.
The characteristic (1) is indispensable for enhancing the display contrast of liquid crystal display elements of SBE mode.
The characteristic (2) is necessary for inhibiting the display from coloring occurrence due to the temperature-dependency of .DELTA.n in the SBE mode, and a clearing point (hereinafter abbreviated to NI point) is desired to be as high as possible. The .DELTA.n value generally exhibits a curve slowly declining from the lower temperature side toward the higher temperature side, but it begins to rapidly lower in the vicinity of the clearing point, and the display color changes when the light path length of birefringence (.DELTA.n.multidot.d) changes notably. Further, when it reaches the NI point, .DELTA.n=0, that is, an isotropic liquid is formed lose which lacks the function necessary for liquid crystal display elements. A practical NI point for display elements is preferred to be 80.degree. C. or higher.
The characteristic (3) is important for enhancing the degree of freedom of the cell thickness of liquid crystal display elements. The SBE mode uses as its display an interference color due to the birefringence effect based upon the light path length (.DELTA.n.multidot.d=constant) of birefringence, differently from the TN mode. Thus, it is preferred to be able to take various An values corresponding to an optional d.
The characteristic (4) is particularly advantageous for reducing the response time in liquid crystal cells of the SBE mode. It is known that as to the response time in the TN mode, the rise time and decay time are both proportional to .eta..multidot.d.sup.2. This relationship is also applicable to the SBE mode.